RSA encryption with public key

Hi! I'm trying to implements TLS handshake in Brightscript and stuck with encrypting premaster secret with RSA. I know we have roRsa object but it only allows to encrypt with private key and in TLS I only have public key from the server. I was trying to implement RSA algorithm manually but modulus is too large to store even in long integer. 

Is there any way to encrypt with RSA public key?